Sources of tight sandstone gas and its migration in the Upper Paleozoic, Yan'an Gasfield

In this paper, the gas source of Yan'an Gasfield in southeastern Ordos Basin is studied through the comparison of carbon isotope data, the analysis of components of natural gas, and the consideration of regional geological background. The results show that the main source rocks in Yan'an Gasfield are coal seams. The carbon isotope inversion occurs in the gas-bearing strata, mainly caused by the mixing of a small amount of oil-type gas in the second number of Shanxi Formation and Benxi Formation or caused by multiple gas fillings in the first number of Shanxi Formation and the eighth number of Shihezi Formation. The fractionation effects of CH₄、CO₂ and iC₄/nC₄ between near-source rocks and far-source rocks are obvious while the differentiation within a layer is not obvious, which is related to the distances of episodic fillings in tight sandstone. Through the study of gas source, it is considered that the overlapping area of high quality coal seam and thick sandstone is the preferred area for future exploration in the region.